power: pmic: rk8xx: Support sysreset shutdown method

Add support for sysreset shutdown for this PMIC. The values were pulled
from the various datasheets, but for now it has only been tested on
the rk817 (for an Odroid Go Advance).

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>